1. Field of the Invention
The present invention is directed in general to communication systems. In one aspect, the present invention relates to a method and system for digitally encoding audio signals used in the broadcast of stereophonic audio signals for use in television and cable broadcasting. In a further aspect, the present invention provides an integrated circuit system for modulating an audio signal using digital BTSC stereo encoding techniques.
2. Related Art
Existing television (TV) sets typically can process a variety of input signal types. However, older TV models may only have a Radio Frequency (RF) input. When using a Set Top Box (STB) to create a video source, a method is needed to get the video signal to the TV. The method used is to modulate the baseband signal to a selectable television channel. For example, North American STBs often modulate to the assigned Channel 3 or Channel 4 Radio Frequencies (RFs). The modulation is typically performed using analog techniques, via discrete components. The prior art approach to implementing the interface to a display requiring an RF modulated input signal has typically been to include these discrete components on a board that interfaces to an output port of a device. This has proven to be a highly expensive endeavor as a relatively large amount of cost is associated with the RF input interface.
There are additional drawbacks associated with conventional STB systems. For example, encoded stereo audio signals transmitted to a conventional STB system are typically received and decoded by stereo audio/video (A/V) components connected directly to the program source. Where A/V components are daisy-chained through a coaxial connector with a conventional STB (cable, television or satellite), only the first device to derive left and right audio in the chain receives stereo audio via the coaxial connector, and all remaining devices that have only decoding circuitry connected only by coaxial cable receive monaural sound. Thus, conventional STB systems provide only limited audio signal processing capabilities for distributing decoded audio signal information to television sets associated with the STB.
An example of a conventional system is a STB cable television receiver system for decoding and distributing BTSC stereo encoded audio signals to associated television sets. In such conventional systems, a radio frequency modulator might be included for modulating the decoded audio signal for output from the STB. Such modulators might include some simple signal processing, such as signal preemphasis processing, but prior STB systems have not included on-chip standards-based stereo encoding of the audio signals prior to distribution to associated audio playback devices. In particular, prior STB systems have not included on-chip BTSC encoding of audio signals. As will be appreciated by those skilled in the art, the BTSC encoding standard for encoding stereo signals was adopted by the Federal Communications Commission (FCC). This standard is also referred to as the Multichannel Television Sound (MTS) Transmission and Audio Processing Requirements for the BTSC System—OET-60. This is an example of a standard which permitted television programs to be broadcast and received with bichannel audio, e.g., stereophonic sound. Similar to the definition of stereo for FM radio broadcast, MTS defined a system for enhanced, stereo audio for television broadcast and reception. Also known as BTSC stereo encoding (after the Broadcast Television System Committee (BTSC) that defined it), the BTSC transmission methodology is built around the concept of companding, which means that certain aspects of the incoming signal are compressed during the encoding process. A complementary expansion of the signal is then applied during the decoding process.
The original monophonic television signals carried only a single channel of audio. Due to the configuration of the monophonic television signal and the need to maintain compatibility with existing television sets, the stereophonic information was necessarily located in a higher frequency region of the BTSC signal, making the stereophonic channel much noisier than the monophonic audio channel. This resulted in an inherently higher noise floor for the stereo signal than for the monophonic signal. The BTSC standard overcame this problem by defining an encoding system that provided additional signal processing for the stereophonic audio signal. Prior to broadcast of a BTSC signal by a television station, the audio portion of a television program is encoded in the manner prescribed by the BTSC standard, and upon reception of a BTSC signal, a receiver (e.g., a television set) then decodes the audio portion in a complementary manner. This complementary encoding and decoding ensures that the signal-to-noise ratio of the entire stereo audio signal is maintained at acceptable levels.
FIG. 1 is a block diagram of the front end portion of an analog BTSC encoding system 100, as defined by the BTSC standard. Encoder 100 receives left and right channel audio input signals (indicated in FIG. 1 as “L” and “R”, respectively) and generates a conditioned sum signal and an encoded difference signal. It should be appreciated that, while the system of the prior art and that of the present invention is described as useful for encoding the left and right audio signals of a stereophonic signal that is subsequently transmitted as a television signal, the BTSC system also provides means to encode a separate audio signal called SAP (Second Audio Program), e.g., audio information in a different language, which is separated and selected by the end receiver. Further, noise reduction components of the BTSC encoding system can be used for other purposes besides television broadcast, such as for improving audio recordings.
System 100 includes an input section 110, a sum channel processing section 120, and a difference channel processing section 130. Input section 110 receives the left and right channel audio input signals and generates a sum signal (indicated in FIG. 1 as “L+R”) and a difference signal (indicated in FIG. 1 as “L−R”). It is well known that for stereophonic signals, the sum signal L+R may be used by itself to provide monophonic audio reproduction and it is this signal that is decoded by existing monophonic audio television sets to reproduce sound. In stereophonic receivers, the sum and difference signals can be added to and subtracted from one another to recover the original two stereophonic signals (L) and (R). Input section 110 includes two signal adders 112, 114. Adder 112 sums the left and right channel audio input signals to generate the sum signal, and adder 114 subtracts the right channel audio input signal from the left channel audio input signal to generate the difference signal.
To accommodate transmission path conditions for television broadcasts, the difference signal is subjected to additional processing than that of the sum signal so that the dynamic range of the difference signal can be substantially preserved as compared to the sum signal. More particularly, the sum channel processing section 120 receives the sum signal and generates the conditioned sum signal. Section 120 includes a 75 μs preemphasis filter 122 and a bandlimiter 124. The sum signal is applied to the input of filter 122 which generates an output signal that is applied to the input of bandlimiter 124. The output signal generated by the latter is then the conditioned sum signal.
The difference channel processing section 130 receives the difference signal and generates the encoded difference signal. Section 130 includes a fixed preemphasis filter 132 (shown implemented as a cascade of two filters 132a and 132b), a variable gain amplifier 134 preferably in the form of a voltage-controlled amplifier, a variable preemphasis/deemphasis filter (referred to hereinafter as a “variable emphasis filter”) 136, an overmodulation protector and bandlimiter 138, a fixed gain amplifier 140, a bandpass filter 142, an RMS level detector 144, a fixed gain amplifier 146, a bandpass filter 148, an RMS level detector 150, and a reciprocal generator 152. The processing of the difference signal (“L−R”) by the section 130 is substantially as described in the Background section of U.S. Pat. No. 5,796,842, which explains that the BTSC standard rigorously defines the desired operation of the 75 μs preemphasis filter 122, the fixed preemphasis filter 132, the variable emphasis filter 136, and the bandpass filters 142, 148, in terms of idealized analog filters. Specifically, the BTSC standard provides a transfer function for each of these components, and the transfer functions are described in terms of mathematical representations of idealized analog filters. The BTSC standard further defines the gain settings, Gain A and Gain B, of amplifiers 140 and 146, respectively, and also defines the operation of amplifier 134, RMS level detectors 144, 150, and reciprocal generator 152. The BTSC standard also provides suggested guidelines for the operation of overmodulation protector and bandlimiter 138 and bandlimiter 124. Specifically, bandlimiter 124 and the bandlimiter portion of overmodulation protector and bandlimiter 138 are described as low-pass filters with cutoff frequencies of 15 kHz, and the overmodulation protection portion of overmodulation protector and bandlimiter 138 is described as a threshold device that limits the amplitude of the encoded difference signal to 100% of full modulation where full modulation is the maximum permissible deviation level for modulating the audio subcarrier in a television signal.
In the past, BTSC stereo encoders and decoders were implemented using analog circuits. Through careful calibration to tables and equations described in the BTSC standard, the encoders and decoders could be matched sufficiently to provide acceptable performance. However, conventional analog BTSC encoders (such as described in U.S. Pat. No. 4,539,526) have been replaced by digital encoders because of the many benefits of digital technology. Prior attempts to implement the analog BTSC encoder 100 in digital form have failed to exactly match the performance of analog encoder 100. This difficulty arises from the fact that the BTSC standard defines all the critical components of idealized encoder 100 in terms of analog filter transfer functions, and prior digital encoders have not been able to provide digital filters that exactly match the requirements of the BTSC-specified analog filters. As a result, conventional digital BTSC encoders (such as those described in U.S. Pat. Nos. 5,796,842 and 6,118,879) have deviated from the theoretical ideal specified by the BTSC standard, and have attempted to compensate for this deviation by deliberately introducing a compensating phase or magnitude error in the encoding process.
Given the processing capabilities of current signal processors, digital implementions of a BTSC encoder can result in the opposite problem of too much accuracy when the digital solution is capable of a far higher signal-to-noise ratio than the analog solution. In this case, the digital encoder does not provide satisfactory performance in regions of operation where noise dominates the operation of the two feedback loops. This results in degradation in the performance of the encoding/decoding system and reduced stereo separation for the encoded signal.
In addition to the complexity of the computational requirements for encoding the stereo signals, such as described above, the ever-increasing need for higher speed communications systems imposes additional performance requirements and resulting costs for BTSC encoding systems, such as where digital circuits requiring high speed clock rates are integrated on-chip with other digital circuits requiring different clock rates. In order to reduce costs, communications systems are increasingly implemented using Very Large Scale Integration (VLSI) techniques. The level of integration of communications systems is constantly increasing to take advantage of advances in integrated circuit manufacturing technology and the resulting cost reductions. This means that communications systems of higher and higher complexity are being implemented in a smaller and smaller number of integrated circuits. For reasons of cost and density of integration, the preferred technology is CMOS. To this end, digital signal processing (“DSP”) techniques generally allow higher levels of complexity and easier scaling to finer geometry technologies than analog techniques, as well as superior testability and manufacturability.
There is a need to provide a digital encoding system for processing stereophonic audio signals in compliance with an audio encoding standard that provides accurately encoded audio signals. Conventionally known systems have attempted to compensate for magnitude or phase errors created by imprecise digital filtering, or have suffered from degraded performance in low frequency operation where the digital encoder has a higher signal-to-noise ratio than the BTSC-specified analog decoder. Further, the nature of existing analog BTSC encoders has made them inconvenient to use with digital equipment such as digital playback devices. A digital BTSC encoder could accept the digital audio signals directly and could therefore be more easily integrated with other digital equipment. Therefore, there is a need for a better system that is capable of performing the above functions and overcoming these difficulties without increasing circuit area and operational power. Further limitations and disadvantages of conventional systems will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.